PRO-PLUS do some that drop straight in as do most manufacturers you just have to get the catalogues out to find them.
I have an amp running a pair of vibe 6x9's in the shelf and have set the head unit bias to provide a fill in soundstage from the front speakers. sounds good enough on a budget.
If you want to do it properly tho, get a good quality component set up, mount the tweeters in the dash and the 6.5's in the doors. With the crossovers supplied set up right, it can and does sound superb, maybe not the loudest but quality audio pleasure.
My mate used an MB Quartz set up....... MMMMMMM nice !